The present invention relates to a thin-film electroluminescent device providing improved optical properties.
In general, AMEL displays are constructed of a thin-film laminar stack comprising a transparent front electrode carrying an alternating current illumination signal, which is typically indium tin oxide deposited on a non-transparent silicon substrate. An electroluminescent phosphor layer is sandwiched between front and rear dielectric layers, all of which is deposited behind the front electrodes. Pixel electrodes are behind the rear dielectric layer, typically consisting of a pad of metal or poly-silicon, positioned at each location a pixel is desired within the phosphor layer. An insulator made of any suitable material, such as SiO2 or glass, is on the pixel electrodes and the rear dielectric layer. The insulator layer is preferably constructed with holes in the insulator layer commonly referred to as VIA for each pixel electrode, to permit the connection of the pixel electrodes to a circuit layer which is deposited on a substrate layer, such as silicon. The circuit layer permits the individual addressing of each pixel electrode. As such, an individual pixel within the electroluminescent layer may be selectively illuminated by the circuit layer permitting a sufficient electrical field to be created between the front electrode and the respective pixel electrode. Normally the AMEL display is fabricated starting with the substrate. One example of an AMEL device is described by Khormaei, U.S. Pat. No. 5,463,279, incorporated by reference herein.
For many applications, such as computer graphics, video, and virtual reality, a multi-color display is desirable. There are several currently accepted techniques to obtain a color display. One such method is the use of spatially patterned filters superimposed over a xe2x80x9cwhitexe2x80x9d screen to provide the three primary colors, such as red, blue, and green. Each of the filters of a pixel provides a respective sub-pixel. An example of a thin-film electroluminescent screen of this type is disclosed by Sun et al., U.S. Pat. No. 5,598,059. However, as the pitch between adjacent pixels becomes increasingly small a greater percentage of the light directed toward and intended for a particular sub-pixel is directed through the filter material overlying an adjacent sub-pixel of a different color. The result is a degradation in the ability to produce accurate colors. A further refinement to increase the color purity includes patterning a substantially non-conductive light absorbing material over the front transparent electrode surrounding the color filters to decrease the light intended for a particular sub-pixel from actually passing through adjacent sub-pixels of a different color.
Tuenge, U.S. patent Ser. No. 08/856,140 discloses an approach to construct a color AMEL device that includes a field-sequential liquid crystal color shutter in series with a broad band white electroluminescent phosphor. The color shutter switches the colors displayed by each pixel using fast transition liquid crystal cells. Unfortunately, the liquid crystal cells absorb a substantial amount of light incident thereon thereby reducing the overall brightness of the display. In addition, the number of different colors that can be displayed during a particular frame is restricted to the switching time of the liquid crystal cells and the electroluminescent light source. Moreover, the liquid crystal cells increase the weight and thickness of the display. Also, the liquid crystal cells are temperature sensitive and reduce the operating temperature range of the device to less that it would have been without the liquid crystal cells.